1. Field of the Invention
The present invention relates to a multi-level power converter which has an improved detection function of an over-current failure.
2. Description of the Background
A two-level inverter has been used as a power converter to change a DC power into an AC power. Recently, needs of large capacity power converter with high voltage are expanded. So-called multi-level inverter equipment having three or more voltage potentials become to be used for example, for a motor drive application.
Three-level inverter equipment is one which outputs three DC voltage potentials. On the other hand, the conventional two-level inverter equipment outputs two DC voltage potentials. In the two-level inverter, switching legs in each of which two switching elements are in-series connected, are connected in parallel to the DC power supply. An AC output is obtained from central points of the switching legs. In three-level inverter, four switching elements connected in-series constituting switching legs are connected to a DC power supply with positive, zero, and negative voltage potentials. An AC output is obtained from central points of the switching legs. The both ends of the switching legs of the three-level inverter are connected to positive and negative terminals of DC power supply, and negative terminal of positive end of the switching element and positive terminal of negative end of the switching element are clamped in zero voltage potential through a clamp diode, respectively. A flywheel diode is anti-parallel connected to every switching element constituting a switching leg.
By making a parallel connection of two switching legs, a single-phase AC output can be obtained. A three phase AC output can be obtained by making a parallel connection of three switching legs at either of two-level or three-level inverter.
In a conventional two-level inverter equipment, when any one of the voltages between positive and negative terminals of switching elements exceeds a threshold voltage level during a period of its ON-state, a method of performing reduction of gate voltage or blocking gate signal has been used for the purpose of over-current protection of a switching element.
That is, when an on-pulse is given to a gate terminal of a switching element in the state of normal operation, the voltage between positive and negative terminals of the switching element decreases to saturation voltage with some delay time. Since there is an internal resistance of the switching element during a period of its ON-state, the voltage between positive and negative terminals of the switching element does not decrease when an over-current flows by a short circuit failure. Therefore, an over-current protection of the equipment can be attained by supervising the voltage of positive and negative terminals of the switching element after the predetermined delay time when an on-pulse is given to the switching element.
A proposal, which applied above mentioned viewpoint to three-level inverter equipment, is made in Japanese Patent Publication (Kokai) No. 2000-354383.
In the equipment shown in the above patent publication, a simplified circuit constitution is obtained because any over-current detectors is not used. But the delay of time is needed to accomplish an over-current protection of the equipment as mentioned above. Further, it is difficult to adjust the delay time. Because a decreasing speed of the voltage between positive and negative terminals of the switching element might be changed or affected by a transient characteristic of the switching element and a load characteristic. Furthermore, in a free-back mode at a middle part of the switching leg, it is difficult to detect an abnormally high voltage between positive and negative terminals of a switching element during a period of its ON-state, because positive current does not flows through the switching element at that time.